Millimeter waves permit generation of images in circumstances where visible and infrared wavelengths cannot since they are not significantly attenuated by common atmospheric constituents. Millimeter wave imaging systems that utilize arrays of detectors take advantage of these properties of millimeter waves in order to generate images of objects obstructed by various adverse environments. Such devices can be employed in contraband detection systems, in equipment for facilitating aircraft landing and take off in fog, and in a variety of highly desirable applications that require generation of images of objects which are not visible or detectable with other techniques. An advantage of devices utilizing arrays of detectors is that images of the entire field of view can be generated in real time, without using mechanical or electronic scanning devices.
Generally, a millimeter wave imaging system functions as follows. A lens or equivalent focusing element is used to focus radiation from the field of view onto a two-dimensional array of imaging elements disposed in the image plane of the lens. Each array element provides a continuous electrical signal responsive to the radiation incident thereon. The output signals of the detectors illustratively are used to drive a video display unit wherein each picture element (pixel) of the displayed image represents radiation from the portion of the image incident on a given detector That is, the image formed by the lens on the detector array is converted to signal outputs from individual detectors, which are mapped one-to-one to corresponding pixels of a video display.
In fields such as contraband detection and aircraft landing equipment, it is desirable to generate images of the highest possible quality. Improvements of image quality are typically associated with increasing display resolution and elimination of interfering background noise.
Since there is a minimum physical distance between detectors in the structure described above, some of the information in an image formed on the detector array is lost. In particular, while that portion of the image radiation that is incident on the central portion of the detector is sensed, around each detector there is a peripheral region where the image information is at least partially lost. A more accurate image would be produced if it were possible to direct radiation from this peripheral region to the central portion of the detector. Also, resolution and image quality would be substantially improved if it were possible to map the radiation imaged on each array element to several pixels, such that the combination of these pixels would represent imaging information from the central and peripheral portions of the detector.
Noise cancellation is a critical issue in the design of any communications and image processing equipment. Not surprisingly, it is an important issue that should be addressed in the design of imaging systems utilizing arrays of millimeter wave detectors.
An effective method of noise cancellation is periodically exposing elements of the array to random background radiation of the environment and subtracting the background signal from the signal which is received from the field of view. Since random noise present in the signal is also present in the background, the noise is eliminated when the background signal is subtracted from the detected signal Additionally, the background signal, also referred to as the comparison load, provides a common reference to all detectors so that effects of gain variations of individual detectors are minimized.
One method of supplying the background signal requires illuminating the field of view with millimeter wave radiation which is amplitude modulated using a square wave, i.e., an on-off signal. The signal, which is detected when the illumination sources are off corresponds to the background noise. When the "on" portion of the square wave is detected, the signal represents the reflected signal combined with background noise. Thus, subtracting the reflected signal that corresponds to the "off" portion of the square wave from the signal that corresponds to the reflected "on" portion yields the reflected signal without noise.
Another method involves continuous switching between the detected signal and the uniform load such as room temperature background. The need to constantly switch back and forth between the detected signal and uniform load creates inefficiencies in the system.
The noise reduction mechanism would be significantly improved if background signal was supplied to the detector arrays synchronously with the detected signal, and without relying on special sources of illumination.